1. Field of the Invention
The present invention relates to a thermal head drive unit for use in a thermal color generating type or thermal ink transfer type recording apparatus or the like employing a thermal head.
2. Description of the Prior Art
In recent years, there has been proposed a variety of thermal head drive units to increase the operation speed and achieve an improvement of effective use of recording energy in a thermal color generating type or thermal ink transfer type recording apparatus.
As a conventional example designed to achieve an efficient use of recording energy, there is a thermal head drive unit disclosed in the Japanese Patent Laid-open Publication No. 43060/1990.
FIG. 10 shows a thermal head drive timing chart of the conventional thermal head drive unit, while FIG. 11 shows the construction of the thermal head driven by means of the conventional thermal head drive unit.
Referring to FIG. 11, the thermal head comprises 2560 units of, heat-generating resistor elements R1 through R2560, transistor pairs (tr1.sub.1, tr2.sub.1) through (tr1.sub.2560, tr2.sub.2560), gates G1 through G2560, latch circuits consisting of D-flip-flop circuits FF1.sub.1 through FF1.sub.2560 and shift resisters consisting of D-flip-flop circuits FF2.sub.1 through FF2.sub.2560. The 2560 units of heat-generating resistor elements R1 through R2560 includes a first block consisting of the odd-numbered resistor elements R1, R3, . . . , R2559 and a second block consisting of the even-numbered resistor elements R2, R4, . . . , R2560.
As shown in FIG. 10, the odd-numbered gates G1, G3, . . . , G2559 are turned on according to a strobe 1 signal, so that the odd-numbered transistor pairs (tr1.sub.1, tr2.sub.1), (tr1.sub.3, tr2.sub.3), . . . , (tr1.sub.2559, tr2.sub.2559) are turned on/off according to the data in the latch circuits FF1.sub.1, FF1.sub.3, . . . , FF1.sub.2559. Thus, the heat-generating resistor elements R1, R3, . . . , R2559 belonging to the first block are supplied with electric power by a power source according to the on/off operation of the transistors thereby to effect energy application to the thermal head. In the same manner, the even-numbered gates G2, G4, . . . , G2560 are turned on according to a strobe 2 signal, so that the even-numbered transistor pairs (tr1.sub.2, tr2.sub.2), (tr1.sub.4, tr2.sub.4), . . . , (tr1.sub.2560, tr2.sub.2560) are turned on/off according to the data in the latch circuits FF1.sub.2, FF1.sub.4, . . . , FF1.sub.2560. Thus, the heat-generating resistor elements R2, R4, . . . , R2560 belonging to the second block are supplied with electric power by the power source according to the on/off operation of the transistors thereby to effect energy application to the thermal head.
In the conventional thermal head drive unit having such a construction as mentioned above, the heat-generating resistor elements belonging to the odd-number and even-number blocks of the thermal head alternately receive electric power in the duty ratio of 50% per every gradation and the alternate drive operation thereof are repeated, The amount of heat generation is controlled by varying the number of electric pulses supplied to the heat-generating resistor elements for recording each dot with successively applying energy without providing any intermission between the time of maximum energy application to one recording line and the time of energy application to the subsequent line thereby to increase the efficiency of energy use.
For the purpose of achieving a higher rate of recording operation, there is proposed a second conventional example in which electric power applied to the heat-generating resistor elements of a thermal head is made as large as possible while reducing the time of supplying electric power to the thermal head (refer to pages 1155 through 1161 of "Color hard copy apparatus for electronic still camera by means of a sublimation type thermal die transfer system" in Vol. 39, No. 12 (1985) of Television Academy Journal).
Generally, a thermal head is used with applying an extremely short pulse electric power thereto, and in such a use condition, each heat-generating resistor element has a life span depending on the total pulse amount applied thereto, the life span normally corresponding to 10.sup.7 to 10.sup.8 pulses (refer to pages 350 to 352 of "Hard Copy Technology" issued by Japan Industrial Engineer-Center (JIEC)). Therefore, the first example of the conventional thermal head drive unit repeats turning on/off of electric power supply to the resistor elements every gradation, resulting in that 255 times in maximum of electric pulse power application to the heat-generating resistor elements is conducted per a recording line. The above also incurs a problem of life span reduction of the heat-generating resistor elements due to a frequent pulse application as well as a problem that, since the thermal head is driven with the resistor elements divided at least into two blocks, it is necessary for the thermal head to provide a special wiring, connector, signal line and the like, resulting in increase of the thermal head drive unit.
Furthermore, in the first conventional thermal head drive unit, there is provided no intermission between the time of the maximum energy application to one recording line and the time of the energy application to the subsequent line so as to increase the energy use efficiency and reduce the recording time. However, the provision of no intermission incurs another problem that heat accumulation in the previous line exerts a significant influence on the current line, which results in blurring an edge of an image or blurring a low-gradation portion and the like. (The above-mentioned phenomenon is referred to as "a tailing phenomenon" hereinafter.) Such a tailing phenomenon is not so conspicuous in a degree of an image quality-formed through recording with a source of an NTSC video signal having the same resolution, however, in recording a high vision image with a high-resolution or such an image having a lot of continuous low-density areas as in computer graphics, degradation of image quality due to the tailing phenomenon cannot be ignored.
In addition, there practically occur a variance in recording density due to deviation in resistance of the heat-generating resistor elements in the thermal head as well as variance in recording density due to environmental temperature change and due to heat accumulation in the thermal head, and therefore, compensation is necessary for the variance in recording density. Such compensation can be effected by increasing or decreasing electric power supply time data input with an application voltage made constant so that each heat-generating resistor element yields the same recording heat energy. However, in the first conventional thermal head drive unit, due to the fact that no intermission is provided between the time of maximum energy application to one recording line and the time of energy application to the subsequent line, it is impossible to eliminate such disadvantages of deterioration of image quality due to the tailing phenomenon, variance in recording density due to deviation in resistance of the heat-generating elements, and variance in recording density due to environmental temperature change and heat accumulation in the thermal head itself.
Otherwise, as described in the second conventional example, when electric power application to the thermal head is increased while reducing the electric power supply time period, the heat-generating resistor elements tend to have such an elevated temperature that causes deformation of a recording paper or an ink sheet due to generated heat. When the elevated temperature of the heat-generating resistor elements exceeds the temperature at which the recording paper starts to be deformed, the heat energy generated from the thermal head is consumed as energy for deforming the recording paper or ink sheet other than energy for transferring the ink. The above fact also causes a problem in efficiency of energy use as well as deformation in dot size order of the recording paper surface corresponding to each heat-generating resistor element, which results in reducing the recording density due to irregular reflection of light and causing deterioration of image quality due to luster loss of the recording paper.